Search Results (7)

In the present manuscript, the influence of reaction time on the hexagonal-to-monoclinic phase transition in hydroxyapatite (HAp) nanofibers synthesized via a low-temperature modified hydrothermal method at 100 °C is investigated. The resulting nanofibers were highly crystalline and stoichiometric, with a Ca/P ratio of approximately 1.67. Comprehensive structural and functional characterization, combining X-ray diffraction with Rietveld refinement, Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, transmission electron microscopy (TEM), and resonance-tracking piezoresponse force microscopy (RT-PFM), was employed to elucidate the role of the non-centrosymmetric monoclinic P2₁/b phase in governing HAp’s structural and piezoelectric properties. The analyses indicated a time-dependent phase evolution from hexagonal (P6₃/m) to monoclinic (P2₁/b), with exclusive formation of the hexagonal phase at 6 h and a clearly dominant monoclinic fraction (73.56%) after 24 h. Nanofibers synthesized for 48 h comprised approximately 98% monoclinic HAp and exhibited elongated morphologies with an average length of 354.82 nm and diameter of 45 nm. RT-PFM measurements confirmed a pronounced piezoelectric response associated with the monoclinic phase, yielding an effective piezoelectric coefficient (d_eff) of 19.85 pm/V. In vitro MTT assays demonstrated that the high monoclinic content did not compromise biocompatibility, as cell viability and cytotoxicity met the requirements of ISO 10993 and ASTM F895 standards. These findings offer new insights into how monoclinic ordering governs the piezoelectric behavior of HAp and suggest a promising strategy for enhancing its performance in biomedical applications. Full article

Hydroxyapatite (HAP) is the most widely accepted biomaterial for repairing bone tissue defects, demonstrating excellent biocompatibility and bioactivity that promote new bone formation. Zirconia (ZrO₂), known for its strength and fracture toughness, is commonly used to reinforce ceramics. In this study, magnesium oxide (MgO) served as a stabilizer for zirconia, resulting in magnesia partially stabilized zirconia (Mg-PSZ). Both Mg-PSZ and HAP were synthesized via coprecipitation and mixed in specific ratios to create composites through a ceramic method involving mixing, compaction, and sintering at 1100 °C. The samples were characterized using techniques such as X-ray powder diffraction (XRPD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS). Structural analyses confirmed the presence of both monoclinic and tetragonal zirconia phases. Besides, the increased wt.% HAP in the composites produced distinct peaks for hexagonal HAP. Crystallite sizes ranged from 27.45 nm to 31.5 nm, and surface morphology was homogeneous with small pores. Elements such as calcium, phosphorus, magnesium, zirconium, and oxygen were detected in all samples. This research also examined microhardness changes in the materials. The findings revealed enhancement in microhardness for the biocomposite with higher zirconia content, 90Mg-PSZ/10HAP sample, with the smallest average pore size, highlighting its potential for biomedical applications. Full article

Apatite pastes derived from eggshell waste (BAp) were implanted onto the calvarial bone of rats, and bone formation was evaluated using X-ray μ-computed tomography (CT) and histological evaluation. BAp was mixed with distilled water to prepare a paste. Monoclinic hydroxyapatite of mineral resources (HAp) was used as a control. A 5 mm diameter PTFE (polytetrafluoroethylene) tube was filled with apatite pastes and implanted in the calvarial bone of 9-week-old Sprague Dawley rats for 8 weeks. A larger radiopaque area, similar to that of native bone, was observed in the BAp paste-implanted specimens than that of HAp paste. The bone mineral density (BMD) value of the BAp paste was significantly higher than that of the HAp paste (p < 0.05). In the histological evaluation, new bone formation was noticed from the calvarial side for both apatite specimens, and HAp remained in the PTFE unlike BAp. The bone mass (BM) value of the BAp paste was significantly higher than that of the HAp paste (p < 0.05). SEM and XRD analyses revealed that BAp was microcrystalline and poorly crystalline. The promotion of new bone formation may contribute to the crystallinity and Mg content of BAp. BAp was found to be useful as a bone regeneration material. Full article

The fabrication of hydroxyapatite (HAp) ceramics prepared by existing conventional sintering requires high-temperature sintering of 1250 °C to 1300 °C. In this paper, the activated metakaolin (MK)/HAp specimens were prepared from varied mix design inputs, which were varied solid mixtures (different amounts of MK loading in HAp) and liquid-to-solid (L/S) ratios, before being pressed and sintered at 900 °C. Phase analysis, thermal analysis, surface morphology, and tensile strength of the specimens were investigated to study the influences of the Al, Si, Fe, Na, and K composition on the formation of the hydroxyapatite phase and its tensile strength. XRD analysis results show the formation of different phases was obtained from the different mix design inputs HAp (hexagonal and monoclinic), calcium phosphate, sodium calcium phosphate silicate and calcium hydrogen phosphate hydrate. Interestingly, the specimen with the addition of 30 g MK prepared at a 1.25 L/S ratio showed the formation of a monoclinic hydroxyapatite phase, resulting in the highest diametrical tensile strength of 12.52 MPa. Moreover, the increment in the MK amount in the specimens promotes better densification when sintered at 900 °C, which was highlighted in the microstructure study. This may be attributed to the Fe₂O₃, Na₂O, and K₂O contents in the MK and alkaline activator, which acted as a self-fluxing agent and contributed to the lower sintering temperature. Therefore, the research revealed that the addition of MK in the activated-HAp system could achieve a stable hydroxyapatite phase and better tensile strength at a low sintering temperature. Full article

Hydroxyapatite (HAp) is a natural hard tissue constituent widely used for bone and tooth replacement engineering. In the present work, synthetic HAp was obtained from calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O) and ammonium phosphate dibasic (NH₄)₂HPO₄ following an optimized microwave assisted hydrothermal method. The effect of pH was evaluated by the addition of ammonium hydroxide (NH₄OH). Hence, different characterization techniques were used to determine its influence on the resulted HAp powders’ size, shape, and crystallinity. By Transmission Electron Microscopy (TEM), it was observed that the reaction pH environment modifies the morphology of HAp, and a shape evolution, from sub-hedral particles at pH = 7 to rod-like nanosized HAp at pH = 10, was confirmed. Using the X-ray Diffraction (XRD) technique, the characteristic diffraction peaks of the monoclinic phase were identified. Even if the performed Rietveld analysis indicated the presence of both phases (hexagonal and monoclinic), monoclinic HAp prevails in 95% with an average crystallite size of about 23 nm. The infrared spectra (FTIR) showed absorption bands at 3468 cm⁻¹ and 630 cm⁻¹ associated with OH⁻ of hydroxyapatite, and bands at 584 cm⁻¹, 960 cm⁻¹, and 1090 cm⁻¹ that correspond to the PO₄³⁻ and CO₃²⁻ characteristic groups. In summary, this work contributes to obtaining nanosized rod-like monoclinic HAp by a simple and soft method that has not been previously reported. Full article

Bioceramic materials, such as hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂, (HAp), can be chemically bound to bone tissue; since they are bioactive and biocompatible. HAp, titanium oxide (TiO₂), and hydroxyapatite/titanium oxide (HAp/TiO₂) nanocomposite nanoparticles were obtained by ultrasound irradiation assisted by sol-gel and co-precipitation methods at different time intervals, using Ca(NO₃)₂•4H₂O, (NH₄)₂HPO₄, and TiOSO₄•xH₂O as calcium, phosphorus, and titanium sources, respectively. HAp, TiO₂, and HAp/TiO₂ nanocomposite powders were characterized by X-ray Diffraction (XRD) and Raman Spectroscopy. The percentages of anatase phase for TiO₂ and of monoclinic and hexagonal phases for HAp were quantified by Rietveld refinement. Furthermore, sample crystallinity in each material was enhanced by increasing the ultrasound irradiation time. The nanoparticle shape was semi-spherical, agglomerated, and between 17 and 20 nm in size. The agglomeration of particles in the samples was corroborated with a Field Emission Scanning Electron Microscope (FESEM). Full article

Calcium hydroxyapatite (HAp) has been crystallized from aqueous solutions in the presence of citrate ions, in two temperature intervals. At lower temperature, where citrate could form the stable 3D-ordered phase Ca-citrate-tetrahydrate (Ca-Cit-TH), only the monoclinic (P2₁/c) HAp polymorph occurs and assumes the shape of fence-like aggregates, built by sharply [010] elongated lamellae dominated by the pinacoid {001}. This pronounced anisotropic growth habit is compared with the usually considered rod-like pseudo-hexagonal occurring in pure aqueous solution growth. The habit change is interpreted by assuming that 2D islands of Ca-citrate-tetrahydrate can be adsorbed as epi- monolayers of thickness d₀₀₁ onto the different growth forms: {001}, {100}, $\left\{10\overline{2}\right\}$, {010}, and $\left\{10\overline{1}\right\}$ of HAp. A comparison is made among the corresponding coincidence lattices, in order to explain on reticular basis the selective adsorption of citrate on the {001} HAp form. The role exerted by the 2D-epitaxially adsorbed Ca-Cit-TH as a “mortar” in the monoclinic HAp “brick” assembly is outlined as well. Full article